Fiber optic circuits generally include a transmitter such as a laser that converts an electrical signal (electrical energy) into a light signal (light energy) and injects the light into an optical fiber. A receiver coupled to the optical fiber converts the light energy into electrical energy after the light signal is transmitted through the optical fiber. A photodiode is one type of semiconductor device that converts light energy into electrical energy. A photodiode may be placed in a reverse bias configuration in which an external voltage potential keeps the photodiode cathode at a higher potential than the photodiode anode. The photodiode substantially blocks the flow of current when under reverse bias. However, a small amount of leakage current may flow from the cathode to the anode.
The photodiode leakage current when under reverse bias can increase due to several factors. First, when light shines on the photodiode, the leakage current increases significantly. FIG. 2 shows the reverse current as a function of optical power for an Optek OP950 photodiode. Increases in optical power lead to proportional increases in leakage current. Second, when no light shines on the photodiode, the leakage current (“dark current” in this case since there is no light) changes greatly with temperature (FIG. 3). FIG. 3 shows the variation of both light current and dark current with temperature for an Optek OP950 photodiode. The light current varies by less than ±10% over the temperature range −40° C. to +100° C. However, the dark current over the same temperature range varies from 1/400 to 1000 times the dark current at room temperature (five orders of magnitude). Thus, relatively small changes in temperature cause relatively large changes in dark current.